Cementitious mixtures, compositions for use in cementitious mixtures, and methods of producing cementitious mixtures

ABSTRACT

Cementitious mixtures, compositions for use in cementitious mixtures, and methods of producing cementitious mixtures wherein the compositions are suitable for modifying or improving certain properties of the cementitious mixtures. The compositions include a superabsorbent polymer (SAP) hydrogel having a macromolecular network structure, and at least one pozzolanic material that is chemically incorporated into the macromolecular network structure of the SAP hydrogel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/154,183, filed Apr. 29, 2015, the contents of which are incorporatedherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Contract No.1454360-CMMI awarded by the National Science Foundation and Contract No.1333468-DGE awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention generally relates to cementitious compositions.The invention particularly relates to cementitious compositionscontaining internal curing agents and mineral admixtures.

Cementitious compositions for forming high-performance concrete (HPC)and ultra-high-performance concrete (UHPC) commonly include variousadditives intended to improve various properties of the compositions andthe concrete they produce. Such additives may include any ingredientsother than Portland cement, water, and aggregate that are added to thecomposition before or during mixing. These additives are referred toherein as admixtures.

Portland-pozzolan blended cement typically contains 20-25 wt. % of apozzolan (also referred to herein as a pozzolanic material), resultingin significant energy and cost savings as less Portland cement can beused. A pozzolan is defined by ASTM C125 (ASTM: American Society forTesting Materials) as a siliceous or siliceous/aluminous material thatwill chemically react with calcium hydroxide (Ca(OH)₂) and/or calciumcations (Ca²⁺) in the presence of water (referred to herein aspozzolanic reaction or reactions) at ordinary temperatures to formcompounds with cementitious properties (including C—S—H which provideslong-term strength and durability to concrete). During setting,pozzolanic reactions result in improved resistance to thermal crackingdue to a relatively low heat of hydration (roughly half that of purePortland cement). Additionally, the pozzolanic reaction products of veryfine (sub-micron) siliceous materials are more efficient at fillingcapillary spaces within the cement paste without the formation of largeexpansive pressures and phases within in the concrete (i.e., typical ofalkali-silica reactions (ASR), which can lead to cracking and decreasedconcrete durability), resulting in a refined pore size distribution(i.e., replacing macropores with microporous material) thus improvingthe strength, durability, and impermeability of the hardened concrete.

Common pozzolans include silica fume and rice husk ash which are bothindustrial waste products that consist of pure amorphous silica in theform of particles. Pozzolanic particles and cementitious mixtures arewell described in U.S. Pat. No. 7,442,248 issued to Timmons on Oct. 28,2008. The contents of this patent are incorporated herein by referencein their entirety. Pozzolanic particles employed in cementitiousapplications are typically in the size range of 50-150 nanometers.Pozzolanic particles smaller in size, for example in the range 10-50nanometers, have higher surface area and hence increased reactivity.Smaller particles are also more likely to react completely and formC—S—H without forming expansive phases typical of ASR. However,particles in this smaller size range are difficult to handle due to thehazardous nature of these particles if inhaled. Additionally, thesesmaller particles typically decrease the workability of the cementitiousmixture, an undesirable result which in turn requires greater pumpingpressures and increases the difficulty of mixture placement.

In an effort to lower their water-to-binder ratios, cementitiousmixtures used to produce HPCs and UHPCs may include internal curingagents. These agents, typically wet porous aggregate or swollensuperabsorbent polymer (SAP) hydrogels, provide a continuous supply ofwater during curing, thus counteracting self-desiccation and reducing orpotentially eliminating autogenous shrinkage and cracking of the cementand achieving a corresponding increase in compressive strength anddurability.

SAP hydrogel particles may swell to as much as 100 times their originalweight in the presence of water as shown in image (a) of FIG. 1. Anexemplary SAP hydrogel may be composed of poly(acrylamide(AM)-acrylicacid(AA)) polymer molecules that are chemically cross-linked toneighboring molecules to form a three-dimensional copolymer network thatwill absorb fluid and swell to form a hydrogel under alkalineconditions. The SAP hydrogels are produced and added to cement as dryparticles, and therefore are relatively easy to transport andincorporate into cementitious mixtures. In addition to their stronghydration performance, SAP hydrogels may reduce thermal expansion andtensile creep and enhance freeze/thaw resistance of the cementitiousmixture. However, upon water release during cement curing, the particlesleave behind voids in the hardened cement, and therefore in theresulting concrete as shown in FIG. 4.

In view of the above, there is an ongoing desire for admixtures capableof modifying or improving the properties and characteristics ofcementitious mixtures and concrete formed therewith.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides cementitious mixtures, compositions foruse in cementitious mixtures, and methods of producing cementitiousmixtures wherein the compositions are suitable for modifying orimproving the properties of the cementitious mixtures and concreteformed therewith. In particular, the compositions include polymershaving particles of at least one pozzolanic material incorporateddirectly into a macromolecular network structure of the polymers.

According to one aspect of the invention, a composition for use as acuring agent in a cementitious mixture includes a superabsorbent polymer(SAP) hydrogel having a macromolecular network structure, and at leastone pozzolanic material that is chemically incorporated into themacromolecular network structure of the SAP hydrogel.

According to another aspect of the invention, a cementitious mixture isprovided that includes an internal curing agent comprising asuperabsorbent polymer (SAP) hydrogel having a macromolecular networkstructure, and at least one pozzolanic material that is chemicallyincorporated into the macromolecular network structure of the SAPhydrogel.

According to another aspect of the invention, a method includesproviding a polymer composition that is end-functionalized with achemically reactive group, incorporating a chemically reactivefunctional group on surfaces of a quantity of particles of a pozzolanicmaterial to form surface-functionalized pozzolanic particles, chemicallyreacting the surface-functionalized pozzolanic particles with theend-functionalized polymer composition to form a plurality ofpolymer-grafted pozzolanic particles, incorporating a cross-linkingagent into the plurality of polymer-grafted particles to form acomposite hydrogel with a macromolecular structure comprising theparticles of the pozzolanic material chemically incorporated therein,and incorporating the composite hydrogel into a cementitious mixture.

According to another aspect of the invention, a method includesproviding a fluid medium having a quantity of particles of a pozzolanicmaterial therein, incorporating a composition on surfaces of theparticles of the pozzolanic material to form surface-functionalizedpozzolanic particles, chemically reacting the surface-functionalizedpozzolanic particles with at least one monomer composition and at leastone chain-transfer agent to form a plurality of polymer-graftedparticles, incorporating a cross-linking agent into the plurality ofpolymer-grafted pozzolanic particles to form a composite hydrogel with amacromolecular structure comprising the particles of the pozzolanicmaterial chemically incorporated therein, and incorporating thecomposite hydrogel into a cementitious mixture.

Technical effects of the compositions, cementitious mixtures, andmethods described above preferably include the capability ofincorporating pozzolanic materials into cementitious mixtures withreduced handling risks, while modifying or improving the beneficialeffects of both the pozzolanic materials and the superabsorbent polymerswithin the cementitious mixture relative to separate additions of thetwo materials.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents superabsorbent polymer (SAP) particles and theirswelling behavior under different conditions. Image (a) showsmacro-scale SAP particles representative of the volume increase from a“dry” dehydrated state (left) to a “wet” water-swollen state (right).Image (b) shows a scanning electron image (scale bar=200 microns)representing an angular morphology of dry SAP particles. Image (c)represents an approximate size distribution of SAP particles commonlyincorporated into mortar mixtures; x-axis reports the sizes as thesieve's mesh opening and the y-axis reports the fraction of total weightthat was left behind on the respective sieve.

FIG. 2 contains four graphs (graphs a through d) representing resultsfrom an investigation of four types of custom-synthesized SAP particles:2 wt. % chemically cross-linked poly(acrylic acid(AA)-acrylamide(AM)copolymer networks containing 17% AA (graph a), 33% AA (graph b), 67% AA(graph c), and 83% AA (graph d). The particles were immersed in purewater, tap water, cement pore solution, and three salt solutionscontaining Na⁺, Ca²⁺, or Al³⁺ cations. The swelling ratio (Q) of the SAPparticles was quantified as a function of immersion time, where Q isdetermined by dividing the total mass of fluid that is absorbed by theparticles during time by the total mass of the dry SAP particle prior toimmersion. Greater Q values indicate greater swelling capacity and thechange in Q values over time indicate the swelling kinetics, with someparticles swelling faster than others (i.e., greater increase in Qduring the same time duration). Compared to their behavior in pure waterand tap water, the particles displayed reduced swelling capacity andaltered kinetics in the presence of Ca²⁺ and Al³⁺ with the greatestreductions in swelling capacity observed for SAP particles containinghigher concentrations of AA in the macromolecular network structure, asshown in graph c and graph d for the 67% and 83% AA, respectively.

FIG. 3 is a graph representing results from an investigation performedon particles of the four custom-synthesized SAPs of FIG. 2, namely, 2wt. % chemically cross-linked poly(acrylic acid(AA)-acrylamide(AM)copolymer networks containing 17% AA, 33% AA, 67% AA, and 83% AA.Particles were immersed in cement pore fluid, which contained a mixtureof ions including Na⁺, K⁺, Ca²⁺, Al³⁺, Fe³⁺. The particles thatcontained the highest concentration of AA displayed rapid deswelling.

FIG. 4 shows images of a cementitious mixture containing SAP aftersetting. Image (a) shows a backscattered scanning electron microscope(SEM) image of the SAP-cement mixture (cross-sectioned) after three daysof curing. Areas formerly occupied by swollen SAP particles appear aslarge voids (outlined in dashes), which are partially filled withportlandite (arrows). Scale bar is 100 microns. Image (b) is an SEMimage of the SAP-mortar mixture (cross-sectioned) after abouttwenty-four hours of curing. Angularly shaped voids left behind fromswollen SAP particles and spherical voids from entrapped air bubbles arevisible.

FIG. 5 schematically represents composite hydrogel-based internal curingagents that chemically incorporate pozzolanic particles into theirpolymer macromolecular network structures. The lines indicate thepolymer, solid dots indicate the chemical cross-links, and large circlesindicate the pozzolanic particles. Image (a) represents a superabsorbentpolymer network (SAP); image (b) represents a superabsorbentpolymer-pozzolan (SAPP) network where polymer is chemically grafted tothe surfaces of pozzolan particles; and image (c) represents a SAPPnetwork with additional ungrafted polymer (SAPP+P).

FIG. 6 shows an optical image of a collection of dry suspensionpolymerized SAP particles that are spherical in shape, demonstrating theability to make regularly shaped particles with tunable diameters in therange of 10 s to 100 s of microns based on selected processingparameters.

FIG. 7 represents preliminary thermogravimetric analysis (TGA) resultsof 60-nm silica particles, including “bare” silica particles, silica-PAMparticles that were functionalized with poly(acrylamide) (PAM) polymerwhich had been covalently attached to (“grafting to”) surfaces of theparticles, and silica-chlorosilane and silica-chlorosilane-azide(functionalization of silica with a chlorosilane molecule and furtherfunctionalization with an azide-containing molecule) particles whichwere products of intermediate synthesis steps and degrade from theirsilica surfaces at high temperatures. Results from thesilica-chlorosilane and silica-chlorosilane-azide indicate that by 700°C., approximately 6% mass had been lost compared to the bare silicaparticles. Results from the silica-PAM particles indicate that by 700°C., an additional 8% mass has been lost compared with the functionalizedsilica particles and this mass can be attributed to PAM that waschemically bound to their silica surfaces. These results indicate thatthe synthesis methods described herein are able to successfully formsilica particles that contain polymers which are chemically bound (orgrafted) to the surfaces of the particles, as compared to polymers thatare only weakly physically bound. The formation of a strong chemicalbond between the polymers and the surface of silica particles ispreferred to form a robust SAPP hydrogel that can withstand (i.e., thatdoes not chemically degrade or dissolve in) the highly alkalineenvironment of cementitious mixtures and function effectively as aninternal curing agent.

FIG. 8 shows images obtained from neutron tomography of two SAPparticles (light grey) surrounded by cement paste after mixing.Shrinking is observed as the SAP particles release water over time,resulting in the formation of vapor-filled void space (dark grey).

FIG. 9 is a representation of autogenous shrinkage measurements of plainmortar and mortar mixtures containing SAP hydrogels with differentratios of AA in their macromolecular network structures. Image (a)reports results using a “fixed-water” (FW) batching method and image (b)reports results using a “fixed-polymer” (FP) batching methods. For theFW batches, for all samples, the total amount of water in the mortarmixture was fixed at a ratio of 0.35 by weight of cement and the dosageof the SAP hydrogels varied for each sample depending on the amount ofSAP required to absorb 5% water by weight of cement. For the FP batches,for all samples, the total amount of SAP in the mortar mixture was fixedat 0.2% by weight of cement and the amount of water in each sample wasvaried based on the maximum swelling capacity of the SAP composition inthat sample (determined from separate swelling capacity experiments inwhich SAP compositions were immersed in cement pore fluid for differentlengths of time). All SAP-mortar mixtures displayed improved performance(i.e., less shrinkage) than the control mixture. The SAP compositionscontaining a higher concentration of AA were the best performers in thefixed-polymer mortar batching method. These results indicate that theoverall macro-scale performance of cementitious mixtures containinghydrogel-based internal curing agents is predominately controlled by thepolymer macromolecular network structure and chemical composition.

DETAILED DESCRIPTION OF THE INVENTION

Composite hydrogel-based internal curing agents and cementitiousmixtures comprising such curing agents are described herein. The curingagents include SAP hydrogels having pozzolanic particles chemicallyincorporated directly into their polymer macromolecular networkstructure. Such curing agents have been observed to exhibit increasedreactivity and impart greater strength and durability to cured cementrelative to conventional internal curing agents, and yet are believed tobe less hazardous in preparation and use.

Certain aspects of the invention are described herein in reference topoly(acrylamide(AM)-acrylic acid(AA)) hydrogels; however, it should beunderstood that the invention is believed to be applicable to any SAPhydrogel. Further, during synthesis of poly(acrylamide(AM)-acrylicacid(AA)) hydrogels, the acrylic acid monomer may be neutralized byadding a base (such as NaOH) to the reaction solution, which convertsthe acrylic acid to sodium acrylate. As such, the termspoly(acrylamide(AM)-acrylic acid(AA)) and poly(sodiumacrylate(PANa)-acrylamide(AM)) are used interchangeably herein.

As previously stated, SAP hydrogels are able to provide a continuoussupply of water during curing of cementitious mixtures, thuscounteracting self-desiccation and reducing or eliminating autogenousshrinkage and cracking of a cement and achieving a correspondingincrease in compressive strength and durability. However, SAP hydrogelparticles may leave behind voids in the hardened cement as shown inFIGS. 4 and 8. In addition, investigations leading to the presentinvention indicated that the absorption capacity of SAP hydrogelparticles in the presence of multi-valent ions can be severely reducedand the kinetics radically changed as shown in FIGS. 2 and 3 to thepoint where the hydrogels actively deswell (i.e., a reduction in theswelling ratio Q is observed with time as fluid is released from thehydrogel) in the presence of multi-valent cations. These characteristicscan be a problem because the aqueous fluid in uncured or “fresh”cementitious mixtures naturally contains a variety of multi-valentcations. As an example, the data plotted in FIG. 2 indicated thatgreater reductions in absorption (compared with pure and tap water)occurred in the presence of Ca²⁺ cations for SAP hydrogel particlescontaining higher concentrations of ionizable functional groups in theirmacromolecular network structures (i.e., the AA segments in thecopolymer structure). In alkaline environments (including cementitiousmixtures), it is known that AA will deprotonate, forming an anionicmoiety that can subsequently form an ionic cross-link with nearbymulti-valent ions. This observed reduction in SAP absorption capacity istherefore consistent with the development of Ca²⁺ ionic cross-linkswithin the copolymer macromolecular network structure, as an increaseddensity of cross-links will cause pre-mature deswelling and restrict theoverall swollen dimensions of the SAP hydrogel particles.

It has been observed that portlandite (a naturally occurring form ofcalcium hydroxide (Ca(OH)₂)) may form in the voids remaining fromdehydrated SAP hydrogel particles within a few hours of setting, asshown in FIG. 4. It was hypothesized that the potential for localconfinement of Ca²⁺ within SAP hydrogels directly contributes to theprecipitation of calcium hydroxide.

The hydrogel-based internal curing agents described herein are intendedto promote the cement curing process not only by providing water to fuelthe curing reaction, but also by facilitating beneficial pozzolanicreactions to convert calcium hydroxide (portlandite) to calcium silicatehydrates (C—S—H; the main product of the hydration of Portland cement)within the cementitious mixtures that further refines the microstructureand improves the strength and durability of the hardened HPC. Toaccomplish this goal, the composite hydrogel-based internal curingagents contain pozzolanic materials chemically incorporated into theirpolymer macromolecular network structures. This is achieved bychemically (covalently) attaching (“grafting”) polymer molecules to thesurfaces of pozzolanic particles and subsequently cross-linking thepolymer molecules to neighboring polymer molecules, forming athree-dimensional superabsorbent polymer-pozzolan composite hydrogelnetworks referred to herein as “SAPP” hydrogels.

While it is well known that the addition of pozzolanic material tocementitious mixtures can enhance the curing reaction and result instronger and more durable concrete at reduced cost, there aresignificant tradeoffs in material processing and properties which canlimit the use of pozzolanic materials in HPC and UHPC. However, suchshortcomings can be overcome or at least partially alleviated bychemically incorporating pozzolanic particles directly into the polymermacromolecular network structures of the SAPP hydrogels, in a mannerthat enables the benefits of pozzolan addition in HPC and UHPC to berealized along with the added benefits of easier material handling,improved processability, and reduced autogenous shrinkage. Inparticular, the benefits to cementitious mixtures by incorporation ofthe SAPP hydrogels therein are expected to be greater than if the twocomponents (pozzolanic particles and SAP hydrogels) were used and addedseparately as discrete additives to cementitious mixtures, as theytypically are currently used in the construction industry. For example,by chemically incorporating the pozzolanic particles into the polymermacromolecular network structures of SAPP hydrogels, the particlesremain in the presence of water (and will thus continue to beneficiallycatalyze the hydration reaction) but they will be much easier and saferto handle and add to the cementitious mixtures relative to looseparticulate pozzolans.

Additionally, as previously discussed, SAP hydrogels tend to attractmulti-valent ions (including calcium ions) which are key reactants inthe cement hydration reaction. These ions may promote the pozzolanicreaction and potentially even “fill in” the void space that wouldtypically remain in the cement following hydrogel deswelling with cementbinder, catalyzed locally by the pozzolanic particles within thehydrogel. Even if the voids are not entirely filled in, the reaction mayenhance strength in the void walls. As seen in FIG. 4, some particlesdehydrate in such a way that the dry polymer may coat the void walls. Inthe case of SAPP hydrogels with pozzolanic particles chemicallyincorporated into their polymer macromolecular network structures, apolymer coating on a void wall resulting from dehydration of the SAPPhydrogel particle may preferentially strengthen the wall region due tothe high local concentration of pozzolanic activity or reactivity (whichgenerally refers to the degree of reaction over time or the reactionrate between a pozzolan and Ca²⁺ or Ca(OH)₂ in the presence of water).

Regardless of whether the voids are entirely filled, chemicalincorporation of pozzolan particles within the SAPP hydrogels is able todecrease the heat of hydration, increase the internal relative humidity(RH), decrease the autogenous shrinkage of the mixture, and/or minimizethe appearance of large pores and microcracks within the hardened cementmicrostructure, thereby increasing the compressive strength anddurability of the concrete. The magnitude of these effects appears to bedirectly dependent on the pozzolan reactivity, which is related to thepolymer grafting density (with higher grafting densities resulting inreduced reactivity). Thus, the grafting density of polymer molecules onthe surface of the pozzolanic particle may be used to control thereactivity of the particle and thus create a tunable response fordifferent types of concrete.

Various methods may be used to produce the SAPP (and SAPP+P, describedhereinafter) hydrogels with controlled variation in pozzolan content andmorphology, polymer-pozzolan grafting density, and polymermacromolecular network structure and chemical composition.

According to one nonlimiting method for producing a SAPP hydrogel,polymer-grafted pozzolanic particles are created with a “grafting to”method utilizing a pre-synthesized uncross-linked polymer compositionthat is end-functionalized so that it can covalently react with thesurface of pozzolanic materials. A polymer composition with a desiredmolecular weight may be synthesized by controlled radical polymerizationfrom free monomer, initiator, and chain transfer agent in solution. Anonlimiting example of a suitable polymer composition includes a silane-or alkyne-terminated poly(AA-AM) copolymer (molecular weight, MW, of500-100,000 g/mol). A chemically reactive functional group (nonlimitingexample, chlorosilane) is incorporated on surfaces of particles of apozzolanic material to form surface-functionalized particles of thepozzolanic material of varying grafting density (from 0.05 to 0.5molecules/nm²), for example, under nitrogen atmosphere at about 70° C.via sonication or reflux in a solvent. The end-functionalized polymercomposition is then chemically reacted with the surface-functionalizedpozzolanic particles, for example, under nitrogen at about 70° C. in asolvent, to form polymer-grafted pozzolanic particles. The graftingdensity of polymer on the surface of the pozzolanic particles can bevaried. To form a SAPP hydrogel, a covalent cross-linking agent is added(nonlimiting example, n,n-methylenebisacrylamide, at a concentration of0.2-10% by weight of polymer) to a plurality of the polymer-graftedpozzolanic particles to create a covalently bonded, three-dimensionalsuperabsorbent polymer-pozzolan composite (SAPP) hydrogel with amacromolecular structure containing particles of the pozzolanicmaterial. Weight percent of pozzolanic particles in the SAPP hydrogelmay range from about 0.5 percent particles by weight of polymer (for arelatively high polymer grafting density on the pozzolanic particlesurface and a relatively high density of covalent cross-links) to about90 percent particles by weight of polymer (for a relatively low polymergrafting density on the pozzolanic particle surface and a relatively lowdensity of covalent cross-links).

According to another nonlimiting method for producing a SAPP hydrogel,polymer-grafted pozzolanic particles are created with a “grafting from”controlled radical polymerization method. The method includes providinga quantity of pozzolanic material in a particulate form in a volume offluid medium. A small molecule with acceptable functionality forreactivity or chain transfer agent is incorporated on surfaces of theparticles of pozzolanic material to form surface-functionalizedparticles of the pozzolan, for example, under nitrogen atmosphere atabout 70° C. via sonication or reflux in solvent. Thesurface-functionalized particles are then chemically reacted with atleast one monomer composition and at least one free chain-transferagent, resulting in a plurality of polymer-grafted pozzolanic particleswith controlled polymer MW of 500-100,000 g/mol and varying graftingdensity (from 0.05 to 0.5 molecules/nm²). A cross-linking agent(nonlimiting example, n,n-methylenebisacrylamide, at a concentration of0.2-10% by weight of polymer) is then incorporated into the plurality ofpolymer-grafted particles to create a covalently bonded,three-dimensional superabsorbent polymer-pozzolan composite (SAPP)hydrogel with a macromolecular structure containing particles of thepozzolanic material. Weight percent of pozzolanic particles in the SAPPhydrogel may range from about 0.5 percent particles by weight of polymer(for a relatively high polymer grafting density on the pozzolanicparticle surface and a relatively high density of covalent cross-links)to about 90 percent particles by weight of polymer (for a relatively lowpolymer grafting density on the pozzolanic particle surface and arelatively low density of covalent cross-links).

As a modification to the above described synthesis method, rather thanreacting the cross-linking agent solely with polymer-grafted pozzolanicparticles, select ratios of pre-synthesized polymer (uncross-linked,with MW ranging from 500-50,000 g/mol) may be mixed with a plurality ofpolymer-grafted particles and the entire mixture cross-linked with thecross-linking agent to yield a SAPP hydrogel network with additionalungrafted polymer incorporated into the polymer macromolecular networkstructure, referred to herein as a SAPP+P hydrogel. The weight fractionrange of pozzolanic materials in SAPP+P hydrogels may be similar to therange previously specified and is ultimately dependent on the molecularweight of the grafted polymer and uncross-linked polymer.

FIG. 5 schematically represents SAP, SAPP, and SAPP+P hydrogel networkscomprising polymers (lines), chemical cross-links (solid dots), andpozzolanic particles (larger circles). Irregularly shaped compositehydrogel particles can be created by drying and crushing the products ofany one or more of the two different synthesis procedures describedabove for SAP, SAPP, and SAPP+P. Regular-shaped particles, for example,spherical-shaped or cylindrical-shaped particles (FIG. 6), can becreated using test tube reaction vessels or suspension polymerizationmethods (for example, adapting the nonlimiting two different synthesisprocedures described above for use in a non-solvent environment, such ascyclohexane).

In general, pozzolanic materials capable of use in embodiments describedabove can be any siliceous or siliceous and aluminous materials that haslittle to no hydraulic reactivity, that is, it does not form a binder inthe presence of water. Non-limiting examples of pozzolanic materials forSAPP and SAPP+P hydrogels include, but not limited to, Class C fly ash,silica fume, metakaolin, rice husk ash, Class F fly ash, slag, and/orcalcined shale. Various sizes of the pozzolanic particles may be used,including particle sizes commonly used in cementitious mixtures. Forexample, silica nanoparticles may have diameters of about 10-100 nm,silica fume particles may have diameters of about 100-1000 nm, and ricehusk ash and other natural pozzolanic particles may have diametersgreater than 1 μm, though pozzolanic particles of other sizes are alsowithin the scope of the invention. The SAPP and SAPP+P hydrogelspreferably comprise pozzolanic particles in an amount of about 75 to 90percent by weight in order to both catalyze a beneficial reaction in theconcrete while still comprising enough polymer to swell and act as aninternal curing agent.

In addition, it should be recognized that a cementitious mixture mayinclude other pozzolanic materials or other additives in addition tothose contained in the SAPP or SAPP+P hydrogels, or may include multipletypes of SAPP or SAPP+P hydrogels each containing different pozzolanicmaterials. Further, the cementitious mixture may include SAPP or SAPP+Phydrogels that comprise only one type of pozzolanic material, or maycomprise more than one pozzolanic material chemically incorporated intoits macromolecular network structure.

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the chemical and physical composition of the SAPhydrogels could differ from that described, other materials could beincorporated into the macromolecular network structure of the SAPhydrogels, and materials and processes/methods other than those notedcould be used. Therefore, the scope of the invention is to be limitedonly by the following claims.

The invention claimed is:
 1. A composition for use as a curing agent ina cementitious mixture, the composition comprising: a superabsorbentpolymer (SAP) hydrogel having a macromolecular network structure; and atleast one pozzolanic material that is chemically incorporated into themacromolecular network structure of the SAP hydrogel, the at least onepozzolanic material being Class C fly ash, silica fume, metakaolin, ricehusk ash, Class F fly ash, slag, calcined shale, or any combinationthereof.
 2. The composition of claim 1, wherein the composition containsmore than one pozzolanic material.
 3. The composition of claim 1,wherein the composition includes 0.5 to 90 percent by weight of thepozzolanic material incorporated into the SAP hydrogel.
 4. Thecomposition of claim 1, wherein the composition further comprisesungrafted polymer incorporated into the macromolecular network structureof the SAP hydrogel.
 5. The composition of claim 1, wherein thecomposition includes 75 to 90 percent by weight of the pozzolanicmaterial incorporated into the SAP hydrogel.